1. Field of the Invention
The present invention relates to a method of detecting a state of synchronization loss in a stepping motor. Specifically, the present invention relates to a method of detecting a state of synchronization loss, in which the state of synchronization loss in an N-phase stepping motor is detected by using a means for applying either control current or voltage to a coil of each phase to thereby drive the N-phase stepping motor and a means for individually measuring a back EMF voltage induced at the coil of each phase.
2. Description of the Related Art
FIG. 1 shows an outline of an electric circuit of a stepping motor 1. In this example, the stepping motor 1 is a two-phase bipolar stepping motor driven by two-phase (phase A and phase B) excitation. The stepping motor 1, as well known, includes: a rotor 3 having a permanent magnet with a multipole magnetization arranged such that two kinds of magnetic poles (N, S) 2 are alternately arrayed in the circumferential direction; at least two stator yokes disposed around the rotor 3 so as to form a multiphase magnetic field with at least two phase; and coils 4 to excite the stator yokes. When pulsed control signals (1a, 1b) are applied to the coils 4 by a driving circuit 5, the rotor 3 is caused to rotate according to a predetermined step unit, wherein the rotation angle and the rotation speed of the rotor 3 can be highly precisely controlled by the number and cycle of pulse signals.
The stepping motor, while advantageous in that thanks to its structure the rotor is free from mechanical contact with the stator thus enabling a long life and in that a large static torque can be produced at the time of excitation, has the problem that the rotor fails to normally rotate when the pulse signal has a short cycle or when the load is large, thus causing a so-called synchronization loss. The stepping motor is frequently used for precisely controlling the rotation angle and speed, and therefore, when the stepping motor loses synchronization, it is necessary to detect the malfunction state immediately and then perform an error handling procedure, such as halt of the rotation or restoration to the normal rotation. To this end, the stepping motor is often equipped with a circuit or a system for detecting a state of synchronization loss and then performing an error handling procedure.
Conventional methods of detecting a state of synchronization loss are described in, for example, Japanese Patent Application Laid-Open No. 2000-166297 and European Patent Application Laid-Open No. EP1460757, in which back EMF voltages (refer to VA, VB in FIG. 1) induced at coils by the rotational behavior of a rotor are measured thereby detecting a state of synchronization loss. Specifically, Japanese Patent Application Laid-Open No. 2000-166297 discloses a method of detecting the state of synchronization loss according to the waveform of a back EMF voltage induced at a coil of at least one phase of an N-phase stepping motor. Also, European Patent Application Laid-Open No. EP1460757 discloses a method of precisely detecting a state of synchronization loss in such a manner that a control signal is halted in each step unit for such a short period of time as not to affect the motor rotation wherein a back EMF voltage induced at a coil is measured during the period of time.
FIGS. 2A and 2B outline the technique of detecting a state of synchronization loss disclosed in European Patent Application Laid-Open No. EP1460757, wherein control signal waveforms (control current waveforms) of a two-phase stepping motor are shown. It is arranged at each of Phases A and B that each control signal is halted at a predetermined timing in each step period (stp) for such a minute period of time (halt time) (t) as not to affect the motor rotation. In this example, the halt time t is provided before and after each of inversion timings (T1 to T6) at which current polarity is changed, and a back EMF voltage is measured during the halt time (t).
The above described methods of detecting a state of synchronization loss may employ an external circuit provided outside, may be incorporated into an IC as an algorithm of a program, or may be installed as a software of a computer, wherein back EMF voltage signals from the coil or data corresponding to the signal are inputted to the electric circuit, the IC or the computer, and if the state of synchronization loss is detected, then a detection signal for indicating synchronization loss is outputted. After the detection signal is outputted, an appropriate error handling procedure is performed, for example, a driving circuit to generate a control signal is to be feedback-controlled according to the detection signal.
The present inventors, et al made a field-investigation into the conventional methods of detecting synchronization loss and found out that it happens with a high probability that the circuit functions improperly such that synchronization loss is judged to occur even prior to actually occurring or fails to be detected in spite of actually occurring. This happens because the motor, when losing synchronization, is caused to rotate and halt repeatedly with short quick steps instead of stopping its rotation, wherein the state of repetition of rotation and halt is significantly fluctuated depending on motor rotation speed, the load, and the positional relation between the stator yoke and the magnetic domain of the multipole-magnetized rotor. Accordingly, the back EMF voltage fluctuation to indicate the state of synchronization loss is diversified, and therefore it is not possible to determine the occurrence of synchronization loss based simply on the assumption that the back EMF voltage fluctuates in a predetermined manner at a predetermined coil of at least one phase as described in Japanese Patent Application Laid-Open No. 2000-166297. Also, it is not possible to successfully achieve a sufficient precision if only the measurement method of the back EMF voltage is innovated as described in European Patent Application Laid-Open No. EP1460757.
Further, the present inventors, et al conducted a simulation for detecting synchronization loss according to the method described in European Patent Application Laid-Open No. EP1460757. Specifically, a gear box was combined with the stepping motor 1 shown in FIG. 1 thereby providing an actuator, and the back EMF voltages (VA, VB) were measured at the time of increasingly applying a torque load to the output gear of the actuator as well as at the time of locking the output gear, that is, at the state of synchronization loss, and the result gained when the algorithm based on the method described in European Patent Application Laid-Open No. EP1460757 was applied was simulated. In this connection, when the output gear was locked, the positional relation between the stator yoke and the magnetic poles 2 was shifted at each test thereby reproducing various states of synchronization loss. The simulation result shows that the stepping motor 1 is judged to lose synchronization when rotating with a torque corresponding to about 70% of the torque value at which the stepping motor 1 actually gets out of synchronization, which means that the maximum torque of the stepping motor 1 cannot be fully utilized. Also, it was found out that the state of synchronization loss is detected with a probability of 85%.
The present inventors, et al have been studying a back EMF voltage and assumed that in order to detect synchronization loss of a stepping motor with an enhanced precision, it is necessary to closely analyze the back EMF voltage and its fluctuation mode at the time of synchronization loss. As a result of the study, it was found out that it is important to judge (or identify) the loss of motor synchronization at the very time of detecting an indication of synchronization loss at any one of phases when the back EMF is analyzed individually at each phase of an N-phase stepping motor. Also, it was found out that the value of back EMF voltage or the fluctuation pattern thereof at the time of synchronization loss are not uniform but diversified.